Silicon wafers useful in the manufacture of semiconductor devices require close scrutiny to detect defects as soon as possible in the manufacturing process. Several apparatus are known in the art for detecting microscopic defects on the surface or near the surface of such devices. One such apparatus utilizes a laser beam that is scanned over the surface of a wafer and includes means for detecting scattered radiation from the wafer surface. The specular reflection is blocked from the detection device by suitable arrangement of the lenses and spatial filters. If the surface of the wafer has an imperfection such as dirt, hills, scratches and the like, the laser beam will be scattered from the imperfection. There are also scattering processes such as Raman scattering, etc., which occur, but the intensity due of the light to such scattering effects is usually negligible. The scattered light from the wafer is collected from about the main axis of the lens and is focused on a detector. The scattered light is converted to electrical impulses which can be counted or in the alternative can be displayed as a bright spot on an oscilloscope. See copending U.S. application Ser. No. 000,813, filed by E. F. Steigmeier et al. on Jan. 4, 1979, now U.S. Pat. No. 4,314,763, issued Feb. 9, 1982, entitled "DEFECT DETECTION SYSTEM" for a detailed description of such a scanning apparatus.
It is difficult to calibrate such a scanning apparatus to predictable dimensions of the observed microscopic defects. The usual method for calibrating such an apparatus is to observe the displayed scan by electron or optical microscopic techniques. Such a procedure of calibration is difficult because it is a cumbersome procedure requiring calibration at locations other than where the scanner is located and is more time consuming. There is a need in the art to provide means for calibrating such apparatus quickly and preferably at the site of the apparatus without the use of optical or electron microscopy.